Two-phase stopped-flow measurement of the protonation of tetraphenylporphyrin at the liquid - Liquid interface

Article abstract of DOI:10.1021/ac9506342

The formation rate of the protonated form of tetraphenylporphyrin (TPP) in a dispersed two-phase system composed of dodecane and aqueous trichloroacetic acid (TCA) was studied by means of a stopped-flow method. The protonation reaction took place at the l

Full text of DOI:10.1021/ac9506342

Anal. Chem. 1996, 68, 1250-1253
Two-Phase Stopped-Flow Measurement of the
Protonation of Tetraphenylporphyrin at the
Liquid-Liquid Interface
Hirohisa Nagatani and Hitoshi Watarai*
Department of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan
The stopped-flow method has been widely employed for kinetic
studies in homogeneous, micellar,6 and microemulsion systems,7
but it has rarely been applied to the study of reaction mechanisms
in liquid-liquid two-phase systems in relevance to the solvent
extraction. This method has, however, potential capabilities to
produce a dispersed two-phase system by mixing rapidly an
aqueous solution with an organic solution, and then to detect the
rapid reactions in the dispersed system, which may attain
equilibria within a few hundred milliseconds. Furthermore, the
reaction at the liquid-liquid interface must be measured directly
by two-phase stopped-flow spectrophotometry, provided that the
reaction is accompanied by a sufficient spectral change for the
detection.
The formation rate of the protonated form of tetraphen-
ylporphyrin (TP P ) in a dispersed two-phase system com-
posed of dodecane and aqueous trichloroacetic acid (TCA)
was studied by means of a stopped-flow method. The
protonation reaction took place at the liquid-liquid
interface, and the diprotonated TP P (H2 TP P 2 +) formed
was adsorbed there. In order to determine the rate-
determining process, changes in absorbance at the ab-
sorption maximum wavelengths of TP P and H2TP P 2+ were
analyzed. The obtained rate constant for the decrease of
TP P in the organic phase, 2 1 ( 2 s-1 , was in agreement
with that for the increase of diprotonated TP P at the
interface, 2 0 ( 3 s-1
. The observed rate constants did
Tetraphenylporphyrin (TPP) is a highly hydrophobic chelate
reagent with a very large molar absorbtivity, e.g., 4.33 × 105 dm3
mol-1 at 416.0 nm in dodecane; hence, it has a bright prospect
for use as an extraction-spectrophotometric reagent. TPP is
known to be protonated by the attack of hydroen ions8,9 and to
produce the diprotonated species (H2TPP2+), which is sparingly
soluble in water.
not show any dependence on concentrations of both TP P
and the acid. The experimental results suggested the rate-
determining step to be the molecular diffusion process
of TP P in the stagnant layer in the organic phase side at
the liquid-liquid interface, and the thickness of the
stagnant layer was estimated as 1 .4 × 1 0 -4 cm.
TPP + 2H+ f H2TPP2+
(1)
The kinetic mechanism of solvent extraction includes, in
general, bulk phase reactions and interfacial reactions. The bulk
phase reactions, e.g., acid-base reactions and metal complexation
reactions in aqueous phases, have been studied extensively by
applying various kinetic methods.1 The interfacial reactions,
however, have been less understood in comparison with the bulk
phase reactions.
When an organic solution containing TPP and an aqueous acid
solution are put into contact, the protonation reaction takes place
at the liquid-liquid interface, and the diprotonated TPP is
adsorbed there.10 The heterogeneous reaction will proceed
through two steps: (1) the molecular diffusion process of TPP
from the organic phase and hydrogen ions from the aqueous
phase to the liquid-liquid interface and (2) the chemical reaction
between TPP and hydrogen ions at the interface. In this case, it
is not necessary to consider the distribution of TPP into an
aqueous phase, since TPP is not soluble in water. The protonation
is accompanied by the distinctive spectral shift at the Soret band
from 416.0 (TPP) to 438.0 nm (H2TPP2+). The large molar
absorbtivity of TPP and the large spectral shift in the protonation
are thought to be well suited for an in situ spectrophotometric
study of the protonation reaction at the liquid-liquid interface.
In this work, the kinetics of the formation of the diprotonated
TPP in the dispersed dodecane-aqueous acid system was studied
Recently, the role of interfacial reaction in metal extraction
kinetics was demonstrated by means of the high-speed stirring
method, employing various extraction systems including chelate
extraction, ion-association extraction, and synergic extraction.2,3
It has been shown in the previous studies, for example, that the
ion-association extraction rate of Fe(II) with a hydrophobic 1,10-
phenanthroline is governed by the formation rate of the 1:1
complex at the liquid-liquid interface,4 while the ion-association
extraction rate of tetrabutylammonium picrate is controlled by the
diffusional mass transfer rate rather than the ion-pairing chemical
reaction itself.5 Now, it is highly necessary to find any kinetic
criteria to distinguish between a mass transfer regime and a
chemical reaction regime.
(6) Robinson, B. H.; White, N. C.; Mateo, C. Adv. Mol. Relax. Processes 1 9 7 5 ,
7, 321-338.
(7) Fletcher, P. D.; Howe, A. M.; Robinson, B. H. J. Chem. Soc. Faraday Trans.
(1) Amdur, I.; Hammes, G. G. Chemical Kinetics; McGraw-Hill, Inc.: New York,
1966; Chapters 5 and 6.
(2) Watarai, H. Trends Anal. Chem. 1 9 9 3 , 12, 313-318.
(3) Freiser, H. Bull. Chem. Soc. Jpn. 1 9 8 8 , 61, 39-45.
(4) Watarai, H.; Sasaki, K.; Sasaki, N. Bull. Chem. Soc. Jpn. 1 9 9 0 , 63, 2797-
2802.
1 1 9 8 7 , 83, 985-1006.
(8) Hibbert, F.; Hunte, P. P. K. J. Chem. Soc. Chem. Commun. 1 9 7 5 , 728-729.
(9) Hibbert, F.; Hunte, P. P. K. J. Chem. Soc. Perkin Trans. 2 1 9 7 7 , 1624-
1628.
(5) Cantwell, F. F.; Freiser, H. Anal. Chem. 1 9 8 8 , 60, 226-230.
(10) Watarai, H.; Chida, Y. Anal. Sci. 1 9 9 4 , 10, 105-107.
1250 Analytical Chemistry, Vol. 68, No. 7, April 1, 1996
0003-2700/96/0368-1250$12.00/0 © 1996 American Chemical Society

by means of two-phase stopped-flow spectrophotometry, in order
to clarify the rate-determining step in the interfacial protonation.
EXPERIMENTAL SECTION
Reagents. meso-R,â,γ,δ-Tetraphenylporphyrin was obtained
from Dojindo Laboratories (Kumamoto, Japan) and dissolved in
dodecane at concentrations ranging from 1.48 × 10-6 to 3.78 ×
10-5 mol dm-3. Dodecane, G.R., used as an organic solvent, was
purchased from Nakarai Tesque (Kyoto, Japan) and was highly
purified by distilling after treatment with a mixture of fuming
sulfuric acid and sulfuric acid. Trichloroacetic acid (TCA), G.R.,
from Nakarai Tesque, was diluted at concentrations from 1.0 ×
10-3 to 0.10 mol dm-3. The ionic strength of TCA solutions was
maintained at 0.10 by the addition of trichloroacetic acid sodium
salt, because the dispersion state of the mixture may be influenced
by the ionic strength of the aqueous solution. In this condition,
the dissociation constant of TCA, pKa, is 0.66.11 Therefore, the
concentration of the hydrogen ion in aqueous solution was
calculated from this value and the total concentration of TCA. The
aqueous solutions were prepared with water distilled and deion-
ized through a Milli-Q system (Millipore, Milli-Q SP.TOC.).
Apparatus. A conventional stopped-flow spectrophotometer
(Unisoku, RSP-601) was used to measure the protonation reaction
in the dispersed systems. The optical cell used for the stopped-
flow measurements was a cylindrical quartz cell of 2.0 min i.d.
(i.e., 2.0 mm maximum optical path length) and 6.0 mm o.d. and
was thermostated at 298.2 ( 0.1 K. A photodiode array UV-
visible detector was used to observe the change in the absorption
spectrum. The radii of the dispersed droplets were measured by
an optical microscope (Nikon, Labphot YF) installed with a CCD
video system. In the present experiment, the dodecane phase
was the continuous phase and the aqueous phase was the disperse
phase. The averaged radius of the dispersed aqueous droplets,
r0, was estimated as (3.3 ( 0.9) × 10-3 cm from the video pictures.
Kinetic Measurements. A dodecane solution containing TPP
and an aqueous solution containing TCA in a couple of gas-drive
syringes were mixed in a volume ratio of 1:1 by a two-jet mixer.
The total volume of the mixture was 0.2 mL for each kinetic run.
The dispersed droplet system of dodecane and aqueous phase
was so rapidly produced in the optical cell that it was sufficiently
agitated for more than ∼1000 ms by the initial mixing energy
without using any surfactant. After a long time standing, the
droplets gradually coaggurated and finally separated into two
phases. The progress of the protonation reaction at the liquid-
liquid interface was followed by acquiring the absorption spectra
from 346.0 to 554.0 nm at 5.0 ms intervals. The apparent first-
order rate constants for the decrease of TPP and the increase of
diprotonated TPP were obtained by analyzing the absorbance
changes at each absorption maximum wavelength. A series of
experiments to examine the dependence of the apparent reaction
rate constants on the TPP and TCA concentrations was carried
out by using 1.0 × 10-2 mol dm-3 TCA solution and 1.90 × 10-5
mol dm-3 TPP solution, respectively.
Figure 1. Logarithmic plot of [H2TPP2+]iSi/[TPP]oVo versus the
concentration of hydrogen ion in an aqueous phase. The concentra-
tion of TPP used in this experiment was 7.62 × 10-6 mol dm-3, and
the total concentration of trichloroacetate was 0.10 mol dm-3
.
Thus, Beer’s law was confirmed, and the molar absorbtivities for
TPP and the diprotonated TPP in the dispersed two-phase system
were determined as 5.85 × 105 and 5.71 × 105 dm3 mol-1 cm-1
,
respectively.
The protonation of TPP at the dodecane-water interface is
written as
TPPo + 2H+ h H2TPP2+
(2)
i
where the subscripts o and i denote the bulk dodecane phase
and the interface, respectively. The diprotonated TPP is expected
to adsorb at the interface, forming an ion-pair with the anion, i.e.,
trichloroacetate ion (TCA-), in the present system. The proto-
nation equilibrium was reached at ∼500 ms. The equilibrium
constant, Ke, for eq 2 can be defined by
[H2TPP2+]iSi/ Vo
[TPP]o[H+]2
Ke )
(3)
where [TPP]o (mol dm-3) and [H2TPP2+]i (mol dm-2) are the bulk
concentration of TPP in a dodecane phase and the interfacial
concentration of the diprotonated TPP at the equilibrium state,
respectively, Si is the total area of the dodecane-water interface,
and V is the volume of the dodecane phase. The values of [H2-
o
TPP2+]iSi/ [TPP]oV could be calculated from the absorbances of
o
both species at 1000.0 ms and the molar absorbtivities. The
obtained values of the concentration ratio were plotted in Figure
1 as a function of the hydrogen ion concentration. The slope of
the linear plot was 1.9, as expected from eq 2, confirming the
formation of the diprotonated TPP. From the intercept, the value
of the equilibrium constant was obtained as 8 × 104 mol-2 dm6.
This value is larger than the 7.7 × 103 mol-2 dm6 determined in
a DMSO-H2O mixture,9 suggesting preferential stabilization of the
diprotonated TPP at the dodecane-water interface.
Beer’s law was examined in the dispersed two-phase system
produced by mixing TPP in dodecane with 0.10 mol dm-3 sodium
trichloroacetate solution or 1.0 × 10-2 mol dm-3 TCA solution. A
linear relationship was obtained between the TPP absorbance and
RESULTS AND DISCUSSION
the TPP concentrations from 1.48 × 10-6 to 3.78 × 10-5 mol dm-3
.
The spectral change in Figure 2 shows the decrease of TPP
in the dodecane phase and the increase of the diprotonated TPP
(11) Martell, A. E.; Smith, R. M. Critical Stability Constants, Vol. 3; Plenum
Press: New York, 1977; p 19.
at the liquid-liquid interface after the mixing. In order to
Analytical Chemistry, Vol. 68, No. 7, April 1, 1996 1251

Table 1. Dependence of the Observed Rate
Constantsa on the Concentrations of TPP and
Hydrogen Ion at 298 K
dependence on
concn of TPPb
dependence on
concn of hydrogen ionc
rate constantd
kd/ s-1 ki/ s-1
rate constantd
[TPP]/
[H+]/
mol dm-3
mol dm-3
kd/ s-1
ki/ s-1
1.5 × 10-6 14.2 ( 2.4 23.6 ( 4.1 2.0 × 10-3 20.9 ( 6.6 19.2 ( 3.3
2.2 × 10-6 20.1 ( 7.8 14.3 ( 3.5 3.0 × 10-3 25.0 ( 5.2 22.9 ( 0.2
2.9 × 10-6 23.0 ( 3.5 13.7 ( 3.6 4.9 × 10-3 19.8 ( 1.0 17.1 ( 1.5
4.2 × 10-6 19.3 ( 1.9 15.8 ( 1.4 6.8 × 10-3 18.6 ( 0.3 16.9 ( 1.1
5.2 × 10-6 19.9 ( 2.0 14.6 ( 0.6 9.6 × 10-3 20.4 ( 0.3 20.5 ( 1.2
7.6 × 10-6 21.3 ( 2.0 21.6 ( 2.3 1.8 × 10-2 17.0 ( 1.7 16.6 ( 0.5
1.9 × 10-5 20.4 ( 0.3 20.5 ( 1.2 4.2 × 10-2 21.6 ( 1.5 22.3 ( 2.0
2.7 × 10-5 16.9 ( 0.6 15.2 ( 1.1 5.6 × 10-2 20.0 ( 2.0 22.6 ( 2.9
3.8 × 10-5 16.5 ( 1.4 15.8 ( 1.7 7.5 × 10-2 22.1 ( 4.3 25.9 ( 5.1
Figure 2. Typical spectral change of the dispersed two-phase
system made by rapid mixing of TPP dodecane solution with TCA
aqueous solution, measured at 50.0 ms intervals. The concentrations
mean
19 ( 3
17 ( 4
mean
21 ( 2
20 ( 3
a
All through the experiment, the total concentration of trichloro-
acetate was 0.10 mol dm-3 b The concentration of TCA was held at
.
of TPP and TCA were 1.90 × 10-5 and 1.0 × 10-2 mol dm-3
,
1.0 × 10-2 mol dm-3 c The concentration of TPP was held at 1.90 ×
10-5 mol dm-3, and that of hydrogen ion was calculated from pKa of
TCA, 0.66. d dd and ki denote the rate constants for the decrease of
TPP and the increase of the diprotonated TPP, respectively.
respectively. The total concentration of trichloroacetate was 0.10 mol
dm-3
.
of diprotonated TPP, fairly consistent with each other. The
consistency between the values of 19 ( 3 and 17 ( 4 s-1 is also
acceptable, considering the poor reproducibility in the kinetic
measurements under the various TPP concentrations. The
averaged value for all data listed in Table 1 was 19 ( 3 s-1. These
results suggest the rate-determining step of the formation of the
diprotonated TPP in the two-phase system to be the molecular
diffusion process rather than the chemical reaction process. The
reaction rate constants between porphyrins and hydrogen ion were
reported to be larger than 106 dm3 mol-1 s-1 in homogeneous
systems.9,12
In this dispersed two-phase system, we have to consider two
types of diffusion process. The diffusion of hydrogen ions in the
aqueous droplet phase to the dodecane-water interface is
governed by spontaneous diffusion. On the other hand, the
transport of TPP molecules in the dodecane phase is attained by
a turbulent flow since the continuous dodecane phase is vigorously
stirred by the mixing energy. However, even when the mixture
was efficiently stirred, a stagnant layer of finite thickness located
on the dodecane side of the interface can be assumed. Therefore,
the one-dimensional random walk model was applied to the
diffusion of hydrogen ion in the aqueous phase and the two-film
theory to the diffusion of TPP in the stagnant layer, respectively,
in order to estimate the time necessary for their diffusion to the
interface.
Figure 3. First-order kinetic profiles at the absorption maximum
wavelengths, λmax ) 416.0 nm for TPP and 438.0 nm for H2TPP2+
Solid lines are the fitting curves obtained by the first-order analysis.
.
determine the increasing rate constants of the diprotonated TPP,
changes in absorbance at the absorption maximum wavelengths
(λmax), 416.0 nm for TPP and 438.0 nm for the diprotonated TPP,
were analyzed. Figure 3 shows the changes in absorbance at each
λmax after subtraction of the base line drift. The absorbance
changes obeyed pseudo-first-order kinetics, since the concentra-
tion of hydrogen ion in the aqueous phase was in large excess.
The rate law can be written as
d[TPP]o d[H2TPP2+]i Si
-
)
) k[TPP]o
(4)
dt
dt
Vo
According to the one-dimensional random walk model, the
diffusion of hydrogen ions from the aqueous phase to the interface
is represented by13
where k (s-1) is the observed rate constant.
The values of the rate constant were evaluated by a curve-
fitting of the absorbance data from 4.0 to 154.0 ms. The observed
rate constants for the decrease of TPP and for the increase of the
diprotonated TPP under the various concentrations of TPP or
hydrogen ion are summarized in Table 1. The results show that
the k values depended on neither concentration of TPP nor
hydrogen ion. From the experiments varying the concentration
of hydrogen ion, the averaged values of rate constants were 21 (
2 s-1 for the consumption of TPP and 20 ( 3 s-1 for the formation
∆h ) 2D t
(5)
x
a
where ∆h, Da, and t are the diffusion distance, the diffusion
coefficient in an aqueous phase, and diffusion time, respectively.
(12) Pasternack, F. R.; Sutin, N.; Turner, H. D. J. Am. Chem. Soc. 1 9 7 6 , 98,
1908-1913.
(13) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons,
Inc.: New York, 1980; Chapter 5.
1252 Analytical Chemistry, Vol. 68, No. 7, April 1, 1996

The devoted time in the transport process can be estimated by
using eq 5 and a few assumptions. If we denote the saturated
interfacial concentration of the diprotonated TPP on the interface
of an aqueous droplet as a, multiplication of 2a by the surface
area of a droplet gives the amount of hydrogen ion required for
the protonation of all TPP molecules adsorbed at the interface.
The relationship between the hydrogen ion concentration in the
+
aqueous phase, cH , and the amount of the hydrogen ion supplied
to the interface for the protonation of TPP can then be described
as follows:
Figure 4. Schematic representation of the protonation of TPP at
the dodecane-water interface. TPP molecule in the stagnant layer
is provided from the continuous dodecane phase by the turbulent flow.
The transport of TPP to the dodecane-water interface is dominated
by only the molecular diffusion. The protonation of TPP proceeds at
the interface, and the diprotonated TPP is adsorbed as the ion-pair
with two trichloroacetate ions.
r0
2
cH
4πr2 dr ) 2a4πr0
(6)
+
∫
r
where r0 and r are the radius of a droplet and the distance from
a center, respectively. The maximum diffusion distance of the
hydrogen ion required for the interfacial protonation can be
is the same as that of the aqueous phase. Provided that the
distribution in droplet size is uniform over the whole system in
calculated from eq 6. A typical value of a for interfacially adsorbed
species is 10-8 mol dm-2 14
.
The minimum value of cH and radius
+
the optical cell, the value of Si/ V was calculated as 777 cm-1 by
of the aqueous droplet in this work are 1.0 × 10-3 mol dm-3 and
3.3 × 10-3 cm, respectively. Thus ∆h (i.e., r0 - r) can be calculated
as 2 × 10-4 cm. If we assume that Da is the same value as the
diffusion coefficient of acetic acid in 0.1 mol dm-3 aqueous solution
o
dividing the sum of interfacial area by the sum of each droplet
volume.
The average droplet radius in the present system was (3.3 (
0.9) × 10-3 cm. The diffusion coefficient of TPP in a dodecane
phase is extrapolated from the values in several alkanes, measured
by Tominaga et al.,17 as 3.5 × 10-6 cm2 s-1. The rate constant of
19 s-1 yields 1.4 × 10-4 cm as the thickness of the stagnant layer.
The values of the thickness previously reported range between 1
× 10-4 and 1 × 10-2 cm.18 Because the present system is
vigorously agitated for at least 1000 ms, the stagnant layer is
thought to become very thin. The mechanism of the protonation
of TPP at the dodecane-water interface is shown schematically
in Figure 4.
In conclusion, the two-phase stopped-flow experiments on the
diluted TPP in a dodecane-aqueous TCA system showed that
the rate-determining step of the protonation of TPP is a molecular
diffusion process of TPP within the stagnant layer in the dodecane
phase with a rate constant of 19 s-1. This value can be thought
of as a limiting maximum value for the solvent extraction rate
constant in a dodecane-water system. In other words, the
interfacial reaction, with rate constant larger than 19 s-1, must be
leveled off in this system, and only a slower reaction can be
assigned to the “true” interfacial chemical reaction. This conclu-
sion has a practical importance in the assignment of the rate-
determining step in the study of solvent extraction kinetics.
Further studies to clarify the role of the liquid-liquid interfacial
reaction in the solvent extraction mechanism are in progress.
at 298 K, 1.20 × 10-5 cm2 s-1 15
, the devoted time in the transport
process is obtained as ∼2 ms. This value is vanishingly shorter
than the time for achieving the protonation equilibrium. As a
result of these estimation, we do not have to consider the diffusion
of hydrogen ions from the bulk aqueous phase to the interface as
the rate-determining step.
In liquid-liquid systems, the two-film theory of Lewis and
Whitman has proved to be useful for dealing with a diffusion
process.16 This theory predicts that two stagnant layers exist in
stirred liquid-liquid systems, one on either side of the interface
with a thickness of δ. In the present system, however, the
stagnant layer model is not applied to the aqueous phase side,
since the interior of the aqueous droplet phase is not stirred. In
the stagnant layer on the organic phase, mass transfer occurs with
only molecular diffusion of species, and Fick’s first law for diffusion
can be applied for the rate analysis. The rate equation for the
diffusion of TPP to the interface is then written as
d[TPP]o Do S
-
)
i [TPP]o
(7)
dt
δ Vo
where Do is the diffusion coefficient of TPP in a dodecane phase.
Equation 7 has the same form as the pseudo-first-order rate
equation, eq 4, in which the term (DoSi/ δV ) is equated with the
o
rate constant k. The thickness of the stagnant layer can be
ACKNOWLEDGMENT
evaluated from the values of k, Do, and Si/ V . The specific
o
The authors thank Professor T. Tominaga of Okayama Uni-
versity of Science for useful suggestions in evaluating the diffusion
coefficient of TPP in dodecane. This work was supported by a
Grant-in-Aid for General Scientific Research from the Ministry of
Education, Science and Culture, Japan (No. 07404042).
interfacial area, Si/ V , can be calculated by using the droplet radii
of the dispersed phase, in which the volume of the dodecane phase
o
(14) Watarai, H.; Satoh, K. Langmuir 1 9 9 4 , 10, 3913-3915.
(15) Landolt-B¨ornstein II Band, 5 Teil; Springer-Verlag: Berlin, 1968; pp 639-
653.
(16) Danesi, P. R. In Principles and Practices of Extraction; Rydberg, J., Musikas,
C., Choppin, R. G., Eds.; Marcel Dekker, Inc.: New York, 1992; Chapter 5.
(17) Takami, K.; Saiki, H.; Tominaga, T.; Hirayama, S.; Scully, A. Proceedings of
the 18th Symposium on Solution Chemistry of Japan; Kusatsu, Nov 3-5, 1995,
pp 126-127.
(18) Skelland, A. H. P. In Science and Practice of Liquid-Liquid Extraction, Vol.
1; Thornton, D. J., Ed.; Oxford University Press: New York, 1992; Chapter
2.
Received for review June 26, 1995. Accepted January 9,
1996.X
AC9506342
X Abstract published in Advance ACS Abstracts, March 1, 1996.
Analytical Chemistry, Vol. 68, No. 7, April 1, 1996 1253